RU2726166C1 - Method of resistance spot welding for connection of sheet steels with applied zinc coating - Google Patents

Abstract

FIELD: technological processes.SUBSTANCE: group of inventions relates to point resistance welding of sheet steels. Thickness of sheets (th) to be welded is 0.5 to 3 mm. At least one of sheets is made of sheet steel (A) of specified composition with applied coating from zinc or zinc alloy and is characterized by ultimate tensile strength (TS) of more than 800 MPa, wherein (TS) × (TEL) > 14000 MPa%, where (TEL) is the total elongation. Spot welding is carried out by resistance of sheet steels to obtain welded seam, characterized by depth of imprint at impression (IDepth) on surface of said steel plate (A), wherein 100 mcm ≤ (IDepth) ≤ 18.68 (Zn) - 55.1, where (IDepth) is expressed in micrometres, and Znis solubility of Zn in sheet steel (A) at 750 °C, expressed in wt. %.EFFECT: depth of possible cracks due to embrittlement is less than 20 mcm, welded joints have high mechanical operational characteristics.21 cl, 7 tbl, 6 ex

Description

The present invention relates to a resistance spot welding method for zinc coated steel sheets to achieve high mechanical resistance of welded joints and to reduce the risk of cracking due to liquid metal embrittlement, in particular when adapting to requirements in the automotive industry.

Zinc or zinc alloy coated sheet steels are very effective in terms of corrosion resistance and are thus widely used in the automotive industry. However, as was established from experience, electric arc welding or spot welding with the resistance of certain steels can cause the appearance of special cracks due to the occurrence of a phenomenon called liquid metal embrittlement (“LMO”) or liquid metal cracking (“LMR”). This phenomenon is characterized by the penetration of Zn in a liquid state along the grain boundaries of the underlying steel substrate under the influence of applied stresses or internal stresses, which are the result of limitation of mobility, thermal expansion, or phase transformations. As it was found, a higher level of stress leads to an increased risk of embrittlement of LMOs. Since the stresses that are present in the joint during welding themselves, in particular, depend on the strength of the base metal, as has been established, welded joints made of steels characterized by increased strength are generally more susceptible to embrittlement of LMOs.

To reduce the risk of embrittlement of LMOs, EP0812647 discloses a method in which electric arc welding is carried out in a shielding gas medium using a metal cored wire containing Cu. However, this process is not adapted for joining thin sheets in the automotive industry.

In addition, JP2006035293 discloses an electric arc welding method using stainless steel wire to produce a weld joint containing more than 25% ferrite and to achieve a tensile strength less than 1.8 times in the weld joint greater than the tensile strength for the base metal. However, in addition to the fact that this process is not adapted to the requirements in the automotive industry, achieving low strength in the weld joint is not desirable.

JP2004211158 also discloses a process for electric contact welding (EX) of pipes, where 3 to 40 ppm are present in the steel. boron. However, the conclusions in this document relate to the specific conditions of the ECS process and cannot simply be inferred for the resistance spot welding process. In addition, the addition of B is not desirable for each grade of high strength steel.

Thus, it is desirable to have a method of manufacturing obtained by using spot welding by resistance of the welds of welded joints of sheets coated with Zn, which would reconcile two conflicting requirements:

- on the one hand, the creation obtained by using spot welding by the resistance of the weld of the welded joint, characterized by high tensile properties according to the measurement in the test for the strength of the lap joint with shear. This feature in the General case is of greater importance with increasing tensile strength for the base metal,

- on the other hand, the creation obtained by using spot welding by the resistance of the weld joint, characterized by high resistance to embrittlement of LMOs, the occurrence of which in the General case occurs to a lesser extent with a decrease in the strength of the base metal.

In particular, it is desirable to have a method where the depth of possible cracks due to embrittlement of the LMO would be maintained at a level of less than 20 micrometers, so as not to impair the mechanical performance of the weld joints. For the same reason, it is also desirable to minimize the number of possible cracks due to the presence of a depth of more than 100 micrometers.

In view of the resolution of such a problem, the invention relates to a resistance spot welding method comprising the following sequential steps:

- obtaining at least two sheet steels having a thickness (th) enclosed in the range from 0.5 to 3 mm, wherein at least one of the sheets is a sheet steel coated with zinc or a zinc alloy (A) characterized by a tensile strength (TS) of more than 800 MPa and a total elongation (TEL) such that (TS) × (TEL)> 14000 MPa -%, where the steel substrate composition (A ) contains when calculated on the mass: 0.05% ≤ C ≤ 0.4%, 0.3% ≤ Mn ≤ 8%, 0.010% ≤ Al ≤ 3%, 0.010% ≤ Si ≤ 2.09%, and 0, 5% ≤ (Si + Al) ≤ 3.5%, 0.001% ≤ Cr ≤ 1.0%, 0.001% ≤ Mo ≤ 0.5% and optional: 0.005% ≤ Nb ≤ 0.1%, 0.005% ≤ V ≤ 0.2%, 0.005% ≤ Ti ≤ 0.1%, 0.0003% ≤ B ≤ 0.005%, 0.001% ≤ Ni ≤ 1.0%, with the remainder being Fe and unavoidable impurities,

- resistance spot welding for at least two sheet steels in order to produce a weld joint characterized by an indentation depth indentation (IDepth) on the surface of said sheet steel (A) such that: 100 μm ≤ (IDepth) ≤ 18 68 (Zn _sol ) - 55.1, where (IDepth) is expressed in micrometers, and where Zn _sol is the solubility of Zn in sheet steel (A) at 750 ° C. in terms of% (mass).

In accordance with one embodiment, the solubility of Zn is such that: Zn _sol = (1 - f _γ ) × (Zn _{α (750)} ) + (f _γ × Zn _{γ (750)} ), where f _γ represents the relative volume fractional the concentration of austenite existing in sheet steel (A) at 750 ° C, which ranges from 0 to 1, where Zn _{α (750)} and Zn _{γ (750)} represent the solubility of Zn, respectively, in ferrite and in austenite of sheet steel (A) at 750 ° C in terms of% (mass.), And where the peritectic temperature (T _per ) of the sheet steel (A) in the presence of Zn is determined using the expression: (T _per ) = 782 + (2.5 Mn) - (71.1 Si) - (43.5 Al) - (57.3 Cr), where (T _per ) is expressed in ° C, and the levels of Mn, Si, Al and Cr are expressed in% (mass.), and:

- Zn _{α (750)} = Zn _{α (Tper)} × (1 + 0.68 × ((Tper) - 750) / (600 - (Tper))), in the case of (T _per ) ≥ 750 ° C, and

- Zn _{α (750)} = Zn _{α (Tper)} × (1 + ((Tper) - 750) / (1160 - (Tper))), in the case of (T _per ) <750 ° C,

Zn _{α (Tper)} = 45.9 - (0.13 Mn) - (17.3 C) + (4.8 Si ² ) - (25.4 Si) - (1.53 Al) - (0 , 73 Cr),

where Zn _{α (Tper)} is expressed in ° C, and the levels of Mn, Si, Al and Cr are expressed in% (mass.),

and where: Zn _{γ (750)} = 2 (- b (750 - Ae1) ² / (Ae3 - Ae1) + b (750 - Ae1)),

in this case: b = 28 / (2 (Ae3 - Ae1) - l), where A _e1 and A _e3 are expressed in ° C and represent the temperatures at which, respectively, the conversion from ferrite to austenite begins and ends.

In accordance with one specific embodiment, f _γ = min {- 0.015 + (1.73 × C) + (0.16 × Mn) - (0.11 × Si) - (0.22 × Al) - (0.056 × Cr); 1}, where the levels of C, Mn, Al, Si, Cr are expressed in% (mass.).

In accordance with one specific embodiment, A _e1 and A _e3 are such that:

And _e1 = 725 - (42.1 Mn) + (27.3 Si) + (9 Al) + (5 Cr),

And _e3 = 923 - (360 C) - (34 Mn) + (37.6 Si) + (131.6 Al) - (24.9 Cr), where the levels of C, Mn, Al, Si, Cr are expressed in % (mass.).

In yet another specific embodiment, Zn _sol is determined in accordance with a method comprising the following sequential steps:

- obtaining sheet steel coated (A) from paragraph 1 of the claims, then

- heat treatment of coated steel sheet (A) at 750 ° C for a period of 170 hours, after which

- cooling of the sheet (A) at a speed of more than 50 ° C / s, then

- measuring the level of Zn in steel at a distance of 1 micron from the steel / coating interface of Zn or Zn alloy.

Preferably, at least one of the steel sheets welded to sheet steel (A) is a sheet of steel coated with zinc or zinc alloy (B), and the sum of the thicknesses of sheets (A) and (B) is not more than than 3 mm.

Still preferably, the sum of the thicknesses of sheets (A) and (B) is not more than 2 mm.

In accordance with one preferred embodiment, zinc or zinc alloy (B) coated steel sheet is steel having a composition comprising: C ≥ 0.04%, Mn ≥ 0.2%, with the remainder being Fe and unavoidable impurities.

According to yet another embodiment, Zn _sol , A _e1 and A _{e3 are} calculated using the values of C _av , Mn _av , Si _av , Al _av and Cr _av ,

while C _av , Mn _av , Si _av , Al _av and Cr _av are, respectively, average levels of C, Mn, Si, Al, Cr, which are measured under the zinc coating of said sheet A over the entire depth in the range from 0 up to 100 microns.

According to a preferred embodiment (IDepth) ≥ 125 μm.

In one preferred mode (IDepth) is measured as a result of the displacement of the welding electrode, and the welding process is stopped at a value (IDepth) comprised between 100 microns and 18.68 (Zn _sol ) of 55.1.

In another preferred mode (IDepth) is measured as a result of the displacement of the welding electrode, and the welding process is stopped at a value (IDepth) comprised between 125 microns and 18.68 (Zn _sol ) of 55.1.

In one preferred embodiment, the welding parameters are selected so that the maximum temperature achieved during welding on the outer part of the imprint zone when pressed at the weld joint seam is such that: T _max (IDia) <Ac3.

The invention also relates to obtained by using spot welding, the resistance of the weld of the welded joint, comprising at least two sheet steels having a thickness (th) enclosed in the range from 0.5 to 3 mm, while at least one of of sheets is a sheet of steel coated with zinc or zinc alloy (A), characterized by a tensile strength (TS) of more than 800 MPa and a total elongation (TEL) such that (TS) × (TEL )> 14000 MPa -%, where the composition of the steel substrate (A) contains, calculated on the weight: 0.05% ≤ C ≤ 0.4%, 0.3% ≤ Mn ≤ 8%, 0.010% ≤ Al ≤ 3%, 0.010% ≤ Si ≤ 2.09%, with 0.5% ≤ (Si + Al) ≤ 3.5%, 0.001% ≤ Cr ≤ 1.0%, 0.001% ≤ Mo ≤ 0.5% and optionally 0.005% ≤ Nb ≤ 0.1%, 0.005% ≤ V ≤ 0.2%, 0.005% ≤ Ti ≤ 0.1%, 0.0003% ≤ B ≤ 0.005%, 0.001% ≤ Ni ≤ 1.0%, while the remainder are Fe and unavoidable impurities, where the indentation depth indentation (IDepth) on the surface of said sheet new steel (A) is such that: 100 μm ≤ (IDepth) ≤ 18.68 (Zn _sol ) - 55.1, where (IDepth) is expressed in micrometers, and where Zn _sol is the solubility of Zn in sheet steel (A ) at 750 ° C when expressed in% (mass.).

In accordance with one preferred embodiment, the resistance spot welding is such that (IDepth) ≥ 125 μm.

In one preferred embodiment, the weld seam obtained using resistance spot welding comprises sheet steel (A) coated with zinc or a zinc alloy, which is characterized by a composition exhibiting a Si content of ≥ 0.5%.

Preferably, the weld seam obtained using resistance spot welding comprises sheet steel (A) coated with zinc or zinc alloy, which is characterized by a composition exhibiting a Si content of ≥ 0.7%.

In accordance with one preferred embodiment, the weld joint obtained using resistance spot welding comprises sheet steel (A) coated with zinc or a zinc alloy, which is characterized by a surface fractional concentration of residual austenite comprised between 7 and 30%.

According to yet another preferred embodiment, the weld joint obtained using resistance spot welding comprises sheet steel (A), which is characterized by an average local concentration of C, Mn, Al, Si and Cr over the entire depth, ranging from 0 up to 100 microns, which differs from the composition in volume for sheet steel (A).

The invention also relates to the use of a welded joint obtained by using spot welding, which has been described in one or more of the embodiments described above, or made in accordance with one or more embodiments described above, for the manufacture of load-bearing parts or parts corresponding to for safety, in motor vehicles.

Now the invention will be described in detail and illustrated by examples without limitation.

First, sheet steels are obtained having a thickness (th) comprised between 0.5 and 3 mm, which is a typical thickness range used in the automotive industry. All of these sheets may have the same thickness or different thicknesses. In the latter case, th _max denotes the greatest thickness of the obtained sheets. These sheets are zinc or zinc alloy coated sheets, with the latter expression referring to coatings where the Zn content is more than 50% (mass). In particular, the coating can be obtained by hot galvanization (“GI”) or by hot galvanization with immediately the following heat treatment at approximately 500 - 570 ° C so as to cause diffusion of iron in the coating and obtain a coating “annealed after hot galvanizing "or" GA ", which contains approximately 7 - 14% Fe. It may also be a zinc or zinc alloy coating obtained by using an electrolytic deposition process or by using a vacuum deposition process. The Zn alloy may also be a Zn-Mg-Al-based coating, such as, for example, a Zn-based coating - 3% Mg - 3.7% Al or Zn - 1.2% Al - 1.2% Mg. At least one sheet (A) of these coated sheets is formed of highly deformable steel having a tensile strength (TS) of more than 800 MPa and a total elongation (TEL) such that (TS) × (TEL)> 14000 MPa -%. The sheet steels embodied in the invention are made using a method that sequentially includes the steps of casting, hot rolling, reeling, optionally intermediate annealing, pickling, cold rolling, continuous annealing and coating. Depending on its mechanical properties, composition and manufacturing process, the microstructure of these steels contains, when expressed in surface fractional concentrations, from 5 to 30% residual austenite. In accordance with the thermomechanical cycle in an industrial production line, these coated steels can be, for example, TRIP steels (with ductility due to martensitic transformation), CFB steels (with carbide-free bainite) or Q - P steels (quenched and redistributed). The composition of highly deformable sheet (A) contains:

- Carbon: in an amount in the range from 0.05% to 0.4% (mass.). In the case of a carbon content of less than 0.05%, the tensile strength will be insufficient, and the stability of the residual austenite that is present in the microstructure of the steel to achieve sufficient elongation will not be obtained. At more than 0.4% C, weldability decreases, since low-viscosity microstructures are formed in the heat-affected zone or in the molten weld zone of the welded joint obtained using resistance spot welding. In one preferred embodiment, the carbon content is in the range of 0.13 to 0.25%, which makes it possible to achieve a tensile strength of more than 1180 MPa.

- Manganese is an element that provides solid-solution hardening, which contributes to obtaining a tensile strength of more than 800 MPa. Such an effect will be obtained in the case of a Mn content of at least 0.3% (mass.). However, with more than 8%, its presence contributes to the formation of a structure characterized by the presence of excessively pronounced segregation bands, which can have a negative effect on the hardenability of welded joints and the consumer properties of an automobile bearing part. Also, the suitability for coating is adversely reduced. Preferably, the manganese content is in the range of 1.4% to 4% to achieve these effects. This makes it possible to achieve satisfactory mechanical strength without increasing the difficulty of industrial steelmaking and without increasing the hardenability of the welded alloys, which would adversely affect the weldability of the sheet of the invention.

- Silicon must be contained in an amount in the range from 0.010 to 2.09% to achieve the desired combination of mechanical properties and weldability: silicon reduces the formation of carbide precipitates during annealing after cold rolling of the sheet due to their low solubility in cementite and due to the fact that this element increases carbon activity in austenite. Thus, the enrichment of austenite by carbon leads to stabilization of austenite at room temperature and to the appearance of behavior corresponding to plasticity due to martensitic transformation (“TRIP”), which means stimulating the conversion of this austenite to martensite under the influence of tension, for example, during molding . In the case of a Si content of more than 2.09%, oxides with high adhesion could form during annealing prior to hot galvanization, which could lead to surface defects in the coating. A silicon content of more than 0.5% contributes to the effective stabilization of austenite, while a Si content of more than 0.7% contributes to the surface fractional concentration of residual austenite concluded ranging from 7 to 30%.

- Aluminum should be contained in an amount in the range from 0.010 to 3.0%. Regarding the stabilization of residual austenite, aluminum has an effect that is relatively similar to that of silicon. However, due to the efficient aluminum promotion of ferrite formation at high temperature, excessive addition of aluminum would increase the Ac3 temperature (that is, the temperature of the complete conversion of steel to austenite during heating) during the annealing stage and would therefore make the industrial process expensive in relation to electrical power required for annealing. Thus, the Al content is less than 3.0%.

- Residual austenite in an amount in the range of 5 to 30% at room temperature is necessary to achieve a high overall elongation. The deformability will be especially high in the case of a surface fractional concentration of residual austenite comprised between 7 and 30%. Sufficient stabilization of austenite is obtained by adding silicon and / or aluminum to the composition in amounts such that: (Si + Al) ≥ 0.5%. In the case of (Si + Al) <0.5%, the fractional concentration of residual austenite could be less than 5%, thus, the ductility and strain hardening characteristics during cold forming would be insufficient. However, in the case of (Si + Al)> 3.5%, suitability for coating and weldability will deteriorate.

- Chromium provides hardening and refinement of the microstructure and makes it possible to control the formation of hypereutectoid excess ferrite during the cooling stage after exposure at maximum temperature during the annealing cycle. In the case of steels that do not contain more than 2.8% Mn, ferrite, if present at a surface fraction of more than 40%, will increase the risk that the tensile strength will be less than 800 MPa . Thus, the level of chromium is in the range from more than 0.001% to less than 1.0% for cost reasons and to prevent excessive hardening.

- Like chromium, molybdenum in an amount ranging from 0.001% to 0.5% is effective in increasing hardenability and stabilizing residual austenite, since this element slows down the decomposition of austenite.

- Steels may optionally contain elements subject to the formation of precipitates in the form of carbides, nitrides or carbonitrides, which thus makes it possible to achieve dispersion hardening. For this purpose, the steel may contain niobium, titanium or vanadium: Nb and Ti in an amount enclosed in the range from 0.005 to 0.1% and V in an amount enclosed in the range from 0.005 to 0.2%.

- Steels may optionally contain nickel in an amount comprised between 0.001% and 1.0% in order to improve viscosity.

- Steel may optionally also contain boron in an amount enclosed in the range from 0.0003 to 0.005%. As a result of segregation at the grain boundary, B reduces grain-boundary energy and, thus, is beneficial for increasing resistance to liquid metal embrittlement.

- The remainder in the composition consists of iron and residual elements resulting from the smelting of steel. In this regard, Cu, S, P, and N are at least considered as residual elements or inevitable impurities. Therefore, their content levels are less than 0.03% for Cu, 0.003% for S, 0.02% for P, and 0.008% for N.

After that, sheet steels coated with zinc or a zinc alloy, at least one (A) of which is characterized by the composition presented above, is superimposed on each other and connected to each other using resistance spot welding. Welding can be homogeneous (i.e., welding sheets (A) with each other) or heterogeneous (i.e., welding sheet steel (A) and one or more coated steel sheets of Zn (B), characterized by a different composition).

Resistance spot welding is a process that combines the application of force and the supply of electric current, both of which act on superimposed sheets welded using copper alloy electrodes. A typical welding sequence includes the following sequential steps:

- the approximation of the electrodes and the application of pressure to the sheets,

- a welding sequence formed by a certain number of periods during which an electric current is successively supplied (“pulses”, periods “under voltage”) or not (periods “without voltage”). During this sequence, the effect of the force on the sheets created by the electrodes is maintained. This effort makes it possible to reduce shrinkage friability and obtain grain refinement.

- the holding period, during which the effect of the force is maintained without applying an electric current, in order to counter the volume expansion of the core of the weld point and its cooling.

- separation of the electrodes from sheets that have been welded together.

At the end of the sequence, the weld joint is characterized by the presence of a core of the weld point from the weld metal of the weld joint, where this core of the weld point was created on the interface between sheet steels. On top of this core of the weld point on the surface of the sheets there is a round recess, called a fingerprint when pressed. Indentation indentation diameter (IDia) corresponds to the diameter of the working end of the welding electrode. The indentation depth (IDepth) can typically be comprised between a few tens of microns and several hundred microns. (IDepth) depends on factors such as:

- welding intensity I,

- the force F applied by the electrodes during welding,

- the duration of the supply of electric current t _I ,

- the initial active resistance of the electrical contact R on the interface between the sheets,

the stress of the plastic flow σ _F at a high temperature of steel,

- stress concentration coefficient K _t due to the geometry of the working end of the electrode,

- sheet thickness th.

If other variables are constant, the larger the values of I, t _I , R, F, K _t , the greater the value (IDepth). The smaller the value of σ _F , the larger the value (IDepth).

The value (IDepth) can either be measured directly on the cut weld seams obtained using resistance spot welding, or measured during the welding process itself in accordance with the explanation of the invention below. The inventors presented as evidence that controlling this fingerprint during indentation within a specific range makes it possible to solve the problem of reconciling sufficient mechanical strength of the weld joint and high resistance to embrittlement of LMOs.

The weld strength of a weld obtained using resistance spot welding is usually measured using a shear lap strength test. Despite its apparent simplicity, this test includes complex modes of urgent recommendations and destruction mechanisms. In practice, the fracture mode is considered as an indicator of mechanical properties and can be classified into categories - by interface, partly by interface and with a break. A tear-out regime where fracture occurs at the periphery of the core of the weld point in the heat-affected zone or in the base metal is desirable since it is associated with the greatest strength and ductility of the weld joint. This mode indicates that the weld joint makes it possible to transmit a high level of force, which, therefore, causes significant plastic deformation in neighboring areas and increases the absorption of strain energy in collision conditions with an obstacle in an accident. On the contrary, the destruction of the weld joint on the interface between the sheets while leaving half the core of the weld point on each sheet occurs at low loads and can affect the redistribution of the load and cause a decrease in the absorbed energy in the welded structure. This mode of fracture along the interface is generally unacceptable for the automotive industry.

The inventors presented as evidence that, in order to ensure the existence of a fracture regime, which is a tear-out regime, the indentation imprint (IDepth) must exceed a minimum value ((IDepth) _min ) of 100 microns. As it is supposed without theory binding, in the case of (IDepth)> (IDepth) _{min the} geometry of the core of the welded point, that is, its features in diameter, height and stress concentration at the interface between the welded sheets, will make it possible to avoid destruction along the interface. Such mechanical properties are even improved and are obtained in a very stable manner when exceeding (IDepth) _{min of} 125 microns.

However, the inventors also presented as evidence that the indentation imprint (IDepth) should be kept below the maximum value (IDepth) _max , depending on the steel composition, in order to avoid cracking due to liquid metal embrittlement. These cracks, which are of the intergranular type and are filled with Zn metal, occur more often on the outer corona of the surface of the indentation upon indentation, which materializes the previous arrangement of the working end of the electrode on the sheet.

This zone undergoes more severe deformation than the central part of the indentation zone during indentation.

Thus, angular cracks in this external zone are more common than central cracks in the imprint region upon indentation. As the inventors established as a result of observations when using a camera for infrared thermography, the area where large angular cracks are present corresponds to the area where the maximum temperature in the welding cycle exceeded Ac3. Cracks appear at the end of the welding sequence, which was determined in the invention, at a temperature of more than 700 ° C. As the inventors established in accordance with their thermal measurements and metallographic observations, in the case of a maximum temperature at the location of the indentation diameter of the indent, i.e., T _max (IDia) of less than Ac3, no deep crack will occur due to embrittlement of the LMO, that is, of any crack having a depth of more than 50 microns. In other words, the occurrence of deep cracks due to embrittlement of the LMO will be avoided if the welding parameters are selected so that the value of T _max (IDia) is less than Ac3. Since this state can be obtained using various combinations of welding parameters, it would be impossible to simply define such combinations in more detail. However, a decrease in I and t _I and an increase in F and R tend to achieve this result.

As the inventors established, based on the observation that cracking due to embrittlement of LMOs occurred at a temperature of more than 700 ° C, measurements of deformations in the critical external zone of the indentation zone during indentation, and measurement of critical deformations in tensile tests carried out in Given the temperature range in the installation for studying the effect of Gleeble temperatures, the occurrence of cracking due to embrittlement of LMOs was avoided or decreased very strongly in highly deformable sheet steel (A) while maintaining the imprint depth upon indentation (IDepth) less than the critical value (IDepth) _max , which depended on the solubility of Zn in the steel substrate (A) in accordance with the expression:

(IDepth) ≤ (IDepth) _max = 18.68 (Zn _sol ) - 55.1, (1)

wherein Zn _sol is the solubility of Zn in sheet steel (A) at 750 ° C. in terms of% (mass).

The value of Zn _sol can be directly measured in accordance with the following method, which includes the following sequential steps:

- heat treatment of sheet steel coated (A) at 750 ° C for a period of time of 170 hours. This step causes diffusion from the Zn metal to the steel substrate to achieve an equilibrium state at the Zn substrate interface.

- cooling the sheet (A) at a rate of more than 50 ° C / s, which makes it possible to maintain high-temperature chemical gradients, up to ambient temperature.

- measuring the level of Zn in steel at a distance of 1 micron from the steel / coating interface of Zn or Zn alloy. This level of Zn content can be measured, for example, using energy dispersive spectrometry of x-rays, which is a technique known per se. Under these conditions, the Zn content measured at a distance of 1 micron is the solubility value of Zn _sol .

Alternatively, the value of Zn _sol can be calculated by taking into account the volumetric or surface fractional concentrations (these two values being identical) of austenite, which is present in steel (A) at 750 ° C, and the solubility of Zn in ferrite and in austenite at a given temperature correspond to the expression:

Zn _sol = (1 - f _γ ) × (Zn _{α (750)} ) + (f _γ × Zn _{γ (750)} ), (2)

where f _γ represents the volumetric fractional concentration of austenite existing in sheet steel (A) at 750 ° C, which is in the range from 0 to 1, and where Zn _{α (750)} and Zn _{γ (750)} represent the solubility of Zn, respectively , in ferrite and in austenite of sheet steel (A) at 750 ° C when expressed in% (mass.).

The solubility of Zn in ferrite at 750 ° C depends on the peritectic temperature (T _per ) of the steel (A) in the presence of a Zn coating (i.e., the temperature above which the Zn saturated steel substrate is in thermodynamic equilibrium with the Zn molten metal fused with Fe and possibly other elements forming a solid substitution solution from a steel substrate) and from the solubility of Zn Zn _{α (Tper)} at peritectic temperature. (T _per ) is determined using the expression:

(T _per ) = 782 + (2.5 Mn) - (71.1 Si) - (43.5 Al) - (57.3 Cr), (3)

where (T _per ) is expressed in ° C, and the levels of Mn, Si, Al and Cr are expressed in% (mass.). Depending on whether or not the value (T _per ) is inferior to a temperature of 750 ° C, Zn _{α (750)} can be calculated in accordance with the expression:

- Zn _{α (750)} = Zn _{α (Tper)} × (1 + 0.68 × ((Tper) - 750) / (600 - (Tper))), in the case of (T _per ) ≥ 750 ° C, (4 )

and

- Zn _{α (750)} = Zn _{α (Tper)} × (1 + ((Tper) - 750) / (1160 - (Tper))), in the case of (T _per ) <750 ° C, (5)

The solubility of Zn at peritectic temperature in ferrite can be calculated in accordance with the expression:

Zn _{α (Tper)} = 45.9 - (0.13 Mn) - (17.3 C) + (4.8 Si ² ) - (25.4 Si) - (1.53 Al) - (0.73 Cr), (6)

where Zn _{α (Tper)} is expressed in ° C, and the levels of Mn, Si, Al and Cr are expressed in% (mass.), the solubility of Zn in austenite is determined using the expression:

Zn _{γ (750)} = 2 (- b (750 - Ae1) ² / (Ae3 - Ae1) + b (750 - Ae1)), (7)

in this case: b = 28 / (2 (Ae3 - Ae1) - l), (8)

where A _e1 and A _e3 are expressed in ° C and represent the temperatures at which, respectively, the conversion from ferrite to austenite begins and ends in equilibrium in the steel substrate of the sheet (A).

A _e1 and A _e3 can either be measured, for example, using conventional techniques based on dilatometric analysis, or can be calculated based on the composition of the steel (A) in accordance with the following expressions:

And _e1 = 725 - (42.1 Mn) + (27.3 Si) + (9 Al) + (5 Cr), (9)

And _e3 = 923 - (360 C) - (34 Mn) + (37.6 Si) + (131.6 Al) - (24.9 Cr), (10)

where A _e1 and A _e3 are expressed in ° C, and the levels of C, Mn, Al, Si and Cr are expressed in% (mass.).

In one particular embodiment, the inventors have provided evidence that the inventive method is advantageously implemented by taking into account the local steel composition immediately below the Zn or Zn alloy coating. In other words, the inventors presented as evidence that instead of taking into account the nominal levels of C, Mn, Si, Al and Cr in expressions (3), (6), (9) and (10) above, the risk cracking due to embrittlement of LMOs was optimally reduced as a result of taking into account the average level of elements: C _av , Mn _av , Si _av , Al _av and Cr _av as measured under zinc or over the entire depth, ranging from 0 to 100 microns , under a coating of Zn or Zn alloy. This embodiment will be particularly appropriate if there is a certain level of segregation over the entire thickness of the sheet or if the composition is modified on the surface using heat treatment carried out on the steel substrate before the coating step. Thus, the average values of local concentrations of C, Mn, Al, Si, and Cr over the entire depth, ranging from 0 to 100 microns, may be different from the composition in the volume of steel and are more appropriate for predicting the occurrence of embrittlement of LMOs. As the inventors have demonstrated, a very large majority of cracks due to embrittlement of LMOs have a depth in the range of 0 to 100 microns, and the average levels of C, Mn, Al, Si and Cr are taken into account within this depth range. These average levels of content can be measured using a technique known per se, such as, for example, optical emission spectroscopy of a glow discharge (OESTR).

In accordance with their experiments, the inventors also presented as evidence that the risk of LMO embrittlement would increase if the sum of the sheet thicknesses (A) and (B) was more than 2 mm. Thus, in order to ensure the presence of an average crack depth due to embrittlement of LMOs in the weld joints obtained by using spot welding with a resistance of less than 40 μm, the sum of the sheet thicknesses (A) and (B) should be less than 3 mm The average depth of mother-in-law can even be reduced to a value of less than 20 microns, in the case of the sum of the thicknesses of the sheets (A) and (B) of not more than 2 mm. Thus, in order to ensure the presence of an average crack depth due to embrittlement of LMOs in the weld joints obtained by using spot welding with a resistance of less than 40 or 20 μm, the sum of the sheet thicknesses (A) and (B) should be, respectively not more than 3 or 2 mm.

As one particular embodiment, highly deformable sheet steel (A) is welded to at least hot dip galvanized sheet steel (B), characterized by a composition comprising: C ≥ 0.04%, Mn ≥ 0.2%, the remainder are Fe and inevitable impurities. In accordance with the experiments conducted by the inventors, the occurrence and severity of embrittlement of LMOs will increase in the case of sheet steel (B), which is steel having a composition comprising levels of C and Mn that are significantly lower than the corresponding characteristics of steel (A), i.e. in case of demonstration by steel (B) of a composition such that: C <0.04% and Mn <0.2%. As expected without wanting to be bound by theory, resistance spot welding creates a molten zone characterized by an intermediate composition between sheet compositions (A) and (B). In the case of a large difference in compositions between sheets (A) and (B), the transformation temperatures for steel (A) and the core of the weld point will be very different, which means that the phase transformation can take place in some areas during the cooling stage of the welding cycle, while other very close zones still will not undergo such a transformation. As it is supposed, since the phase transformation occurs with changes in volume, this situation creates an increase in transient stresses, which turn out to be unfavorable from the point of view of liquid metal embrittlement. Thus, given the weakening role of embrittlement of LMOs, provided that the compositions embodied in the invention are present, it is preferable to avoid offering steel (B) a composition characterized by a low C content (C <0.04%) and a low Mn content (Mn < 0.2%).

The invention will now be illustrated using the following examples, which are in no way limiting.

Example 1

Three steels have been proposed, referred to under the designations SA, SB and SC. The composition of these steels when expressed in% (mass.), With the remainder being iron and the inevitable elements resulting from steelmaking, are presented in Table 1. Among the residual or inevitable elements for these steels, the S content is less than 0.003 %, and the content of P is less than 0.014%. The sheets are coated with an electrolytically deposited Zn coating having a thickness of 16 μm.

Table 1 also presents the solubility of Zn Zn _sol at 750 ° C, calculated in accordance with the expressions (2 - 10) above, together with the mechanical tensile properties for steels (UTS: tensile strength, TEL: total elongation) as measured in accordance with ISO standard ISO 6892-1 published in October 2009, presented in table 1. The thickness of sheet steel SA - SB is 1 mm, the thickness of sheet steel SC is 1.2 mm.

Table 1. Composition (% (mass.)), Zn solubility at 750 ° C and tensile properties for steels

C Mn Si Al Si + Al Cr Mo Nb, V, Ti Znsol (%) UTS (MPa) UTS × TEL (MPa -%) SA 0.20 1.65 1,63 0.06 1,69 0,03 0.002 Nb: 0.001
Ti: 0.006
V: 0.001 9.09 820 23780 SB 0.20 1,64 0.80 0.68 1.48 0.04 0.002 Nb: 0.001 15.9 805 23200 SC 0.219 2.07 1.48 0,034 1,514 0.35 0.002 Nb: 0.002b 9.44 1233 16645

Sheet steels were spot-welded with resistance using an electrode having a working end diameter of 6 mm and an alternating electric current at 50 Hz with a force of 350 daN. In accordance with the intensity and times of the duration of spot welding by resistance, various impression depths were obtained upon indentation. For example, for steel (SA), the welding conditions SA1, SA2, SA3 ... make it possible to obtain different imprint depths when pressed.

The weld strength of welded joints obtained by using spot welding resistance was measured by testing the strength of the lap joint with shear in accordance with ISO standard 14273. The test result will be considered satisfactory if there is no observed fracture along the interface.

After that, the welds were etched in a dilute HCl solution containing an inhibitor, so as to remove the coating from zinc, cutting and polishing in such a way as to determine and measure with an increase in the range from 2.5 to 100x with respect to the possible presence of cracks due to liquid metal embrittlement. The depth of the cracks was measured in relation to ten welds of the welded joints in such a way as to produce an average crack depth when calculated on one face of the weld joint. It is desirable to obtain an average crack depth of less than 20 microns.

The observation results are presented in table 2 together with the calculation of the value (IDepth _max ) in accordance with the expression (1) above.

Table 2. Presentation of the characteristics of cracks caused by embrittlement of LMOs in the weld joints and the mode of fracture of the weld joint in tests for shear strength of the lap joint with shear

Underlined values: not relevant to the invention.

Example 2:

The sheet steel SC of Example 1, as a result of resistance spot welding, was assembled with SD steel, the composition of which is shown in Table 3, with the remainder being iron and inevitable impurities resulting from steel smelting.

Table 3. The composition of the steel SD (% (mass.))

C Mn Si Al Ti SD 0.0018 0,083 0.006 0,053 0.04

SD steel was obtained in the form of hot galvanized steel sheets having various thicknesses of 0.8 mm or 1.9 mm, with a Zn coating of 12 microns. Thus, welded joints were made, obtained using spot welding with resistance, with a total thickness of 2 or 3.1 mm. The average crack depth on the surface of the weld joint obtained by using spot welding resistance, for sheet steel SC was determined in the same manner as explained in example 1.

Table 4. Crack depth in weld joints having different aggregate thicknesses

Thus, a decrease in the total thickness below 3 or even 2 mm makes it possible to fabricate welded joints obtained by using spot welding with resistance, characterized by a reduced average crack depth.

Example 3

A resistance spot welding machine, including a pneumatic clamp at 10 kN and a power electric transformer at 63 kVA, was equipped with a sensor capable of detecting the vertical displacement of the welding electrode. The difference between the position of the electrode before and after welding works corresponds to the indentation depth (IDepth). In parallel, a high-speed survey camera was provided, and a mark was made on the welding electrode in such a way as to act as an indicator of recording the position of the electrode during welding. As it was established after comparing with the values of the indentation prints according to the measurement with respect to the welds themselves, two methods (sensor and high-speed shooting camera) were able to ensure accurate value acquisition (IDepth).

Under the conditions explained in Example 1, an SB steel was proposed which was welded using welding cycles that ceased when (IDepth) reached a value that was either greater than the maximum imprint depth upon indentation of IDepth _max corresponding to a given steel composition or less than she is. The results are presented in table 4.

Table 4. Presentation of crack characteristics due to embrittlement of LMOs, and fracture mode in weld joints, interrupted in accordance with the size of the impression upon indentation

Underlined values: not relevant to the invention.

Thus, the offset of the electrode can be used in such a way as to fulfill the function of an input variable, indicating the need to stop welding to ensure, thereby, the quality of the weld joint.

Example 4

An infrared camera equipped with an InSb detector was used in such a way as to measure the surface temperature of the sheet at the immediate periphery of the electrode, that is, at the location of the indentation diameter of the impression (IDia). During the welding cycle, the temperature rises to the maximum T _max (IDia) before cooling immediately after the cessation of the supply of electric current. Thus, it is possible to compare the measured temperature T _max (IDia) with the Ac3 temperature of the steel. The tests were carried out with respect to the SA-SB steels, which were mentioned in Example 1, using various parameters in such a way as to obtain different impression depths when pressed. The presence of angular cracks due to embrittlement of the LMO was evaluated, having a depth of more than 50 μm. For steels SA and SB, Ac3 values are 930 ° С and 965 ° С, respectively.

Table 5. Representation of temperature and crack characteristics at the periphery of the indentation region during indentation

Underlined Tmax values (IDIA): not relevant to the invention.

Thus, according to the observation, when choosing the welding parameters so that the value of T _max (IDia) is less than Ac3, the occurrence of deep angular cracks due to embrittlement of LMOs is avoided.

Example 5

SC steel in the uncoated state was heated in such a way as to modify the composition on its surface within a depth of 100 microns, and then Zn was coated by electrolytic deposition so as to obtain a coating identical to that of Example 1. Medium the levels of C _av , Mn _av , Si _av , Al _av and Cr _av under zinc over the entire depth comprised between 0 and 100 microns under the Zn coating were measured using glow emitting optical emission spectroscopy. The coated steel sheet was welded in an SC4 state similar to the SC3 state of Table 2.

We measured the average number of cracks having a depth of more than 100 μm, calculated on one weld joint obtained using spot welding resistance. These results are presented in table 6 together with the calculation result (IDepth _max ) in accordance with the above expressions (1 - 10), taking into account either the nominal composition of sheet steel C or the average composition over the entire depth, in the range from 0 to 100 microns, under a coating of Zn.

Table 6. The number of cracks depending on the composition in volume or on the surface

Steel Weld seam The maximum depth of the impression upon indentation (IDepth _max ) (μm) The average number of cracks due to embrittlement of LMOs, calculated per weld joint, having a depth of more than 100 μm SC SC3 121 3.4 SC4,
modified composition on the surface within a depth of 100 microns 176 1,5

Based on the identical nominal composition of SC steel, experiments SC3 and SC4 were carried out. The use of only the nominal composition of SC steel for these experiments in calculating the maximum indentation depth during indentation would lead to a prediction of the identity of the experimental results. However, in the SC4 experiment, the number of large cracks is much reduced in comparison with that in the SC3 experiment. This demonstrates that in the case of a change in composition on the surface of the steel, the maximum impression depth during indentation should be calculated based on the average composition over the entire depth of 100 microns, and not on the basis of the steel composition in volume.

Example 6

Zn coated steel sheets having a thickness of 1.6 mm and characterized by the composition of SC steel in Table 1 were welded using resistance spot welding in such a way as to create a weld joint SC4.

In addition, the SC sheet steel having 1.6 mm in thickness was welded with the SD sheet steel having 1.6 mm in thickness and characterized by the composition of Table 3, so as to create a weld joint SC5. The number of cracks in the two weld joints is presented in table 7.

Table 7. The number of cracks depending on the difference in composition between sheet steel

Weld seam The average number of cracks caused by embrittlement of LMOs per one weld joint SC4 6 SC5 15

Since SD sheet steel is characterized by very low levels of C and Mn, the difference in composition between C and Mn between SC and SD is large, and the risk of cracking increases.

As can be said by comparison, the number of cracks in a uniform weld of SC4 weld is reduced.

Thus, due to its high mechanical properties, weld joints obtained by using resistance spot welding, formed from deformable high-strength steels made in accordance with the invention, can be advantageously used for the manufacture of load-bearing parts or safety-related parts in mechanical vehicles .

Claims (84)

1. The method of spot welding resistance, comprising the following sequential steps: the use of at least two sheet steels having a thickness (th) comprised between 0.5 and 3 mm, wherein at least one of the sheets is a sheet steel (A) coated with zinc or a zinc alloy characterized by a tensile strength (TS) of more than 800 MPa and a total elongation (TEL) such that (TS) × (TEL)> 14000 MPa%, while the composition of the sheet steel (A) contains, based on the weight : 0.05% ≤ C ≤ 0.4%, 0.3% ≤ Mn ≤ 8%, 0.010% ≤ Al ≤ 3%, 0.010% ≤ Si ≤ 2.09%, moreover, 0.5% ≤ (Si + Al) ≤ 3.5%, 0.001% ≤ Cr ≤ 1.0%, 0.001% ≤ Mo ≤ 0.5% and optional 0.005% ≤ Nb ≤ 0.1%, 0.005% ≤ V ≤ 0.2%, 0.005% ≤ Ti ≤ 0.1%, 0,0003% ≤ B ≤ 0,005%, 0.001% ≤ Ni ≤ 1.0% the remainder is Fe and inevitable impurities, spot welding resistance of the specified at least two sheet steels to obtain a weld characterized by an indentation depth indentation (IDepth) on the surface of the specified sheet steel (A), such that: 100 μm ≤ (IDepth) ≤ 18.68 (Zn _sol ) - 55.1, where (IDepth) is expressed in micrometers, and Zn _sol is the solubility of Zn in sheet steel (A) at 750 ° C. in terms of wt.%. 2. The method according to p. 1, in which Zn _sol = (1 - f _γ ) × (Zn _{α (750)} ) + (f _γ × Zn _{γ (750)} ), where f _γ represents the relative volumetric fractional concentration of austenite present in sheet steel (A) at 750 ° C, which is in the range from 0 to 1, Zn _{α (750)} and Zn _{γ (750)} represent the solubility of Zn in ferrite, respectively and in austenite of sheet steel (A) at 750 ° C. when expressed in wt.%, while the peritectic temperature (T _per ) of the steel sheet (A) in the presence of Zn is determined using the expression: (T _per ) = 782 + (2.5 Mn) - (71.1 Si) - (43.5 Al) - (57.3 Cr), where (T _per ) is expressed in ° C, and Mn, Si, Al and Cr are expressed in wt.%, and Zn _{α (750)} = Zn _{α (Tper)} × (1 + 0.68 × ((T _per ) - 750) / (600 - (T _per ))), in the case of (T _per ) ≥ 750 ° C, and Zn _{α (750)} = Zn _{α (Tper)} × (1 + ((T _per ) - 750) / (1160 - (T _per ))), in the case of (T _per ) <750 ° C, Zn _{α (Tper)} = 45.9 - (0.13 Mn) - (17.3 C) + (4.8 Si ² ) - (25.4 Si) - (1.53 Al) - (0 , 73 Cr), where Zn _{α (Tper) is} expressed in ° C, and Mn, Si, Al and Cr are expressed in wt.%, wherein Zn _{γ (Tper)} = 2 (- b (750 - A _e1 ) ² / (A _e3 - A _e1 ) + b (750 - A _e1 )), b = 28 / (2 (A _e3 - A _e1 ) - l), where A _e1 and A _e3 are expressed in ° C and represent the temperatures at which, respectively, the conversion of ferrite to austenite begins and ends. 3. The method according to p. 2, in which f _γ = min {- 0.015 + (1.73 × C) + (0.16 × Mn) - (0.11 × Si) - (0.22 × Al) - (0.056 × Cr); 1}, where C, Mn, Al, Si, Cr are expressed in wt.%. 4. The method according to any one of paragraphs. 2 or 3 in which And _e1 = 725 - (42.1 Mn) + (27.3 Si) + (9 Al) + (5 Cr), And _e3 = 923 - (360 C) - (34 Mn) + (37.6 Si) + (131.6 Al) - (24.9 Cr), where C, Mn, Al, Si, Cr are expressed in wt.%. 5. The method according to p. 1, in which Zn _sol is determined in accordance with a method comprising the following sequential steps: obtaining coated steel sheet (A), heat treatment of said coated steel sheet (A) at 750 ° C. for a period of 170 hours, after which sheet cooling (A) at a rate of more than 50 ° C / s, after which measuring the level of Zn in steel at a distance of 1 μm from the steel / coating interface of Zn or Zn alloy. 6. The method according to any one of paragraphs. 1-5, in which at least one of the steel sheets welded to sheet steel (A) is a sheet steel (B) coated with zinc or zinc alloy, the sum of the thicknesses of sheets (A) and (B) is no more than 3 mm. 7. The method according to p. 6, in which the sum of the thicknesses of the sheets (A) and (B) is not more than 2 mm 8. The method according to any one of paragraphs. 6 or 7, in which the aforementioned sheet steel (B) coated with zinc or zinc alloy is steel, characterized by a composition containing, wt.%: C ≥ 0.04 Mn ≥ 0.2, the remainder being Fe and inevitable impurities. 9. The method according to any one of paragraphs. 2-4, in which Zn _sol , A _e1 and A _{e3 are} calculated using the values of C _av , Mn _av , Si _av , Al _av and Cr _av , wherein C _av , Mn _av , Si _av , Al _av and Cr _av are respectively the average levels of C, Mn, Si, Al, Cr, which are measured under a zinc coating of said sheet A at a depth in the range from 0 to 100 μm . 10. The method according to any one of paragraphs. 1-9, in which (IDepth) ≥ 125 μm. 11. The method according to any one of paragraphs. 1-10, in which (IDepth) is measured by offsetting the welding electrode, and the welding process is stopped at a value (IDepth) in the range from 100 μm to 18.68 (Zn _sol ) - 55.1. 12. The method according to p. 11, in which (IDepth) is measured by offsetting the welding electrode, while the welding process is stopped at a value (IDepth) in the range from 125 μm to 18.68 (Zn _sol ) - 55.1. 13. The method according to any one of paragraphs. 1-10, in which the welding parameters are selected so that the maximum temperature achieved during welding on the outer part of the imprint zone when indented at the weld, is such that: T _max (IDia) <A _c3 , wherein (IDia) is the location of the indentation diameter of the fingerprint. 14. A spot welded joint comprising at least two sheet steels having a thickness (th) ranging from 0.5 to 3 mm, wherein at least one of the sheets is a sheet steel (A) coated with zinc or zinc alloy, characterized by a tensile strength (TS) of more than 800 MPa and a total elongation (TEL) such that (TS) × (TEL)> 14000 MPa%, with the sheet composition steel (A) contains, based on the weight: 0.05% ≤ C ≤ 0.4%, 0.3% ≤ Mn ≤ 8%, 0.010% ≤ Al ≤ 3%, 0.010% ≤ Si ≤ 2.09%, moreover, 0.5% ≤ (Si + Al) ≤ 3.5%, 0.001% ≤ Cr ≤ 1.0% 0.001% ≤ Mo ≤ 0.5% and optional 0.005% ≤ Nb ≤ 0.1%, 0.005% ≤ V ≤ 0.2% 0.005% ≤ Ti ≤ 0.1%, 0,0003% ≤ B ≤ 0,005%, 0.001% ≤ Ni ≤ 1.0% while the remainder are Fe and inevitable impurities, wherein the depth of the indentation impression (IDepth) on the surface of said steel sheet (A) is such that: 100 μm ≤ (IDepth) ≤ 18.68 (Zn _sol ) - 55.1, where (IDepth) is expressed in micrometers, and Zn _sol is the solubility of Zn in sheet steel (A) at 750 ° C. in terms of wt.%. 15. The weld according to claim 14, in which (IDepth) ≥ 125 μm. 16. The welded joint according to claim 14 or 15, wherein said at least one of the sheets, which are sheet steel (A) coated with zinc or a zinc alloy, is characterized by a composition containing Si ≥ 0.5%. 17. The weld according to claim 16, wherein said at least one of the sheets is a sheet steel (A) coated with zinc or a zinc alloy, characterized by a composition containing Si ≥ 0.7%. 18. Welded joint according to any one of paragraphs. 14-17, wherein said at least one of the sheets of sheet steel (A) coated with zinc or a zinc alloy is characterized by a surface fractional concentration of residual austenite ranging from 7 to 30%. 19. Welded joint according to any one of paragraphs. 14-18, wherein said at least one of the sheets of sheet steel (A) coated with zinc or a zinc alloy is characterized by an average local concentration of C, Mn, Al, Si, and Cr at a depth within from 0 to 100 μm, differing from the basic composition of sheet steel (A), while the aforementioned average value of local concentrations of C, Mn, Al, Si and Cr is measured by optical emission spectroscopy of a glow discharge (OESTR). 20. The use of the resistance spot welding method according to any one of paragraphs. 1-13 for the manufacture of load-bearing parts in motor vehicles. 21. The use of a welded joint according to any one of paragraphs. 14-19 for the manufacture of load-bearing parts in motor vehicles.